For many years, silicon dioxide (SiO2) has been used in semiconductor substrates for components such as transistor gate dielectrics and capacitor dielectrics. However, as circuit components have reduced in size, the electrical performance characteristics of SiO2 results in undesirable effects such as increased leakage current. Controlling leakage current to maintain high speed and low power performance presents a challenge when older-generation dielectrics such as SiO2 are used in the fabrication of newer integrated circuit geometries.
Newer processes, especially those that use fabrication geometries less than 65 nm have begun to include high dielectric constant (“high-k”) insulators in semiconductor fabrication. Some chipmakers now rely on high-k dielectrics, especially for 45 nm and smaller process geometries. Replacing SiO2 gate dielectrics with high-k dielectrics is important to achieve smaller device geometries while controlling leakage and other electrical performance criteria.
While the use of high-k dielectrics allows for smaller scaling of integrated circuit components such as transistor gate dielectrics, challenges arise in their fabrication. Certain metal and rare earth oxides such as zirconium oxide, titanium oxide, hafnium oxide, tantalum oxide, aluminum oxide, yttrium oxide, and lanthanum oxide are known to provide desirable characteristics when deposited as thin films yet present challenges during the fabrication process such as incompatibilities between process chemistries, extended deposition cycle times, and less than desired deposition uniformity.
A wide variety of methods and related apparatus exist to provide a thin film on a substrate such as a semiconductor. Some methods form a thin film on a substrate by utilizing a surface reaction on the semiconductor, such as vacuum evaporation deposition, molecular beam epitaxy, different variants of Chemical Vapor Deposition (CVD) (including low-pressure CVD, organometallic CVD and plasma-enhanced CVD) and Atomic Layer Epitaxy (ALE). ALE is also referred to as Atomic Layer Deposition (ALD).
ALD is a method of depositing thin films on a surface of a substrate through the sequential introduction of various precursor species. A conventional ALD apparatus may include a reaction chamber including a reactor and substrate holder, a gas flow system including gas inlets for providing precursors and reactants to a substrate surface and an exhaust system for removing used gases. The growth mechanism relies on the adsorption of a precursor on the active sites of the substrate and conditions are preferably maintained such that no more than a monolayer forms on the substrate, thereby self-terminating the process. Exposing the substrate to a first precursor is usually followed by a purging stage or other removal process (e.g., an evacuation or “pump down”) wherein any excess amounts of the first precursor as well as any reaction by-products are removed from the reaction chamber. The second reactant or precursor is then introduced into the reaction chamber at which time it reacts with the first precursor, and this reaction creates the desired thin film on the substrate. The reaction terminates when all of the available first precursor species adsorbed on the substrate has been reacted with the second precursor. A second purge or other removal stage is then performed which rids the reaction chamber of any remaining second precursor and possible reaction by-products. This cycle can be repeated to grow the film to a desired thickness.
One of the recognized advantages of ALD over other deposition processes is that it is self-saturating and uniform, as long as the temperature is within the ALD window (which is above the condensation temperature and below the thermal decomposition temperature of the reactants) and sufficient reactant is provided to saturate the surface in each pulse. Thus, neither temperature nor gas supply needs to be perfectly uniform in order to obtain uniform deposition.
ALD is further described in Finnish patent publications 52,359 and 57,975 and in U.S. Pat. Nos. 4,058,430 and 4,389,973. Apparatus for implementing these methods are disclosed in U.S. Pat. Nos. 5,855,680, 6,511,539, and 6,820,570, Finnish Patent No. 100,409, Material Science Report 4(7)(1989), p. 261, and Tyhjiotekniikka (Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp. 253-261.
Different film materials have been deposited employing ALD. Known materials for use in ALD include binary oxides such as Al2O3, HfO2, ZrO2, La2O3 and Ta2O5. Various ternary oxides are also well known materials for use in ALD and include HfZrO, HfAlO and HfLaO. As discussed previously, selection of the appropriate material for use in high-k dielectric applications requires consideration of the impact of the deposited substance on the particular substrate and circuit environment, as well as considerations over process chemistry. In the case of ALD of HfLaO, a known Hf-precursor is HfCl4 and a known La-precursor is La(THD)3. Due to the hygroscopic nature of La2O3, ozone (O3) is often used instead of H2O as an oxidant in prior art processes, but unfortunately, both the HfCl4/O3 process and the La(THD)/O3 process are highly sensitive to even small changes in the ozone present. In some instances, use of ozone also results in less than desired uniformity of the deposited oxide film. Further, managing two different oxidation chemistries complicates the deposition process when it is desirable that a single oxidizer (such as ozone) could be used in a manner to obtain efficient and consistent deposition results, regardless of the type of metal precursor being used in the deposition process.
Plasma discharges can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing solid materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
Plasmas can be generated in various ways including current discharge, radio frequency (RF) discharge, and microwave discharge. Current discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Parallel plates are typically used for electrostatically coupling energy into plasma. Induction coils are typically used for inducing current into the plasma. Microwave discharges are achieved by directly coupling microwave energy through a microwave-passing window into a discharge chamber containing a gas. Microwave discharges are advantageous because they can be used to support a wide range of discharge conditions, including highly ionized electron cyclotron resonant (ECR) plasmas.
ALD systems have used plasma-based approaches to create oxidant gasses such as ozone. In one common configuration, Dielectric Barrier Discharge (DBD) ozone generators create ozone (O3) from oxygen (O2) that is provided as a feedgas to a corona discharge source. Referring to FIG. 5, a simplified DBD ozone generator cell 500 is illustrated. Typically, dry feedgas oxygen 530 is passed through a gap 505 formed between electrodes 510A, 510B, which are in turn energized by a high voltage source such as an alternating current (AC) voltage source 560. The voltage produced by the source 560 can reach several thousand volts, depending on the configuration of the generator. Alternatively one of the electrodes may be at ground potential, and the other electrode energized to a high voltage. A dielectric material 520A, 520B, is interposed between the energized electrodes 510A, 510B and the feedgas 530. When high voltage at low or high frequencies is applied to the electrodes 510A, 510B, ozone 550 is produced in the feedgas by micro-discharges taking place in the gap 505 and distributed across the dielectric 520A, 520B. The geometry of the gap and the quality of the dielectric material vary by the ozone generator manufacturer. Of note, DBD devices can be fabricated in many configurations, typically planar, using parallel plates separated by a dielectric or in a cylindrical form, using coaxial plates with a dielectric tube between them. In a common coaxial configuration, the dielectric is shaped in the same form as common fluorescent tubing. It is filled at atmospheric pressure with either a rare gas or rare gas-halide mix, with the glass walls acting as the dielectric barrier. Common dielectric materials include glass, quartz, ceramics and polymers. The gap distance between electrodes varies considerably, from 0.1 mm to several cm, depending on the application. The composition of the feed gas is also an important factor in the operation of the ozone generator.
High-performance ozone generators that use the DBD principle require nitrogen in the feed gas to obtain optimum performance and consistent ozone generation. The formation of ozone involves a reaction between an oxygen atom, an oxygen molecule and a collision partner such as O2, N2 or possibly other molecules. If the collision partner is nitrogen, the nitrogen molecules are able to transfer their excitation energy, after impact, to the oxygen molecules resulting in dissociation. Some of the excited nitrogen radicals that are formed may also dissociate oxygen or react with nitrogen oxides to liberate oxygen atoms. Many different forms of nitrogen-oxygen compounds may be produced during the process—NO, NO2, N2O, and N2O5, have been measured in the output DBD-type ozone generators. Some manufacturers have focused efforts to reduce or eliminate altogether the presence of certain N—O species from the output ozone stream of their ozone generators, as in some instances, aggressive corrosion of gas lines and welds from N—O compounds in the ozone stream may occur. In conventional ozone generators, control over the presence and type of N—O compounds in the output stream of ozone generators is lacking, and a need exists to be able to monitor and/or actively control the formation and generation of such compounds.
Thus, a need exists for a method for depositing a dielectric film on a substrate with reduced throughput times and with enhanced deposition uniformity. What is also needed is a system to monitor and/or control nitrogen-oxygen compounds created in an oxidizer generator such as an ozone generator.